Calling a .Net Web Service with Kerberos (or NTML) authentication from Oracle PL/SQL using sys.utl_h

We are calling a .Net webservice from our oracle database using the sys.utl_http package and  we have also tested with the sys.utl_dbws package, this works fine when there is no security on the .Net webservice.
However, we would like to use sys.utl_http or sys.utl_dbws to call a .Net webservice with Kerberos- or NTLM authentication (we're currently struggling with this).

Any hints on how to solve this, is it possible?

Thanks!

Peter